In conventional methods for producing a glass substrate for a magnetic disk, there has been carried out such a method for processing a glass substrate into a concentric circular form, i.e., a doughnut-like form; conducting a lapping process for polishing it with use of separating abrasive grains on a metallic surface plate; and conducting further a polishing process wherein the surface having been lapped is polished with use of a polishing pad made of synthetic resin and separating abrasive grains having a finer grain size.
Washing was generally conducted between the lapping process and the polishing process, and an etching process for removing foreign substances on the surface of the glass substrate for a disk was not conducted. As a result, abrasive grains having a larger grain size produced during the lapping process and metallic particles resulted from the metallic surface plate for lapping were left in the polishing process which was conducted subsequent to the lapping process, and these caused occurrence of flaws in the surface of the glass substrate for a disk and a damage to the polishing pad.
As an example for the etching process, there has been known a method disclosed in, for instance, Japanese unexamined Patent Publication No. 230621/1995. The main purpose of this method is to reduce the depth of flaws existing in an inner circumferential area of the glass substrate to thereby increase the mechanical strength of the glass substrate. Accordingly, there is a condition that the depth of the glass substrate surface by the etching be deeper than 3 .mu.m. As another example, there is a case that an etching process is conducted as a texture (minute projections and recesses in a magnetic disk surface) forming technique to improve friction sliding characteristics to the magnetic head.
In this method, however, a polishing process is not used after the etching process. Accordingly, abnormal projections which exceed the height of the texture around the projections are formed at a masked portion in the glass substrate surface for a disk, due to deposition of dust or the like. When a magnetic disk produced by using such glass substrate is rotated at a high speed such as 3000 rpm and a magnetic head is floated above the disk at a height of 350 .ANG. for instance, there was a possible problem of causing head-crushing of the magnetic head or breakage of a portion in the magnetic disk surface. In this case, it is necessary to suppress the depth by the etching to be less than 350 .ANG. for instance. However, such a small amount (depth) of etching was insufficient to remove the abrasive grains and the like, and a liquid phase etching which was usually carried out made it difficult to control the depth by the etching.